The present invention relates to a transmission adapted to produce multistep speed changes for various tractors for use in agricultue, civil engineering or the like.
Transmissions for giving multistep speed variations require many speed change shafts, so that the speed change shafts must be ingeniously supported and housed in a casing. Accordingly, the input shaft for receiving the power from the prime mover is usually arranged coaxially with a second speed change shaft in end-to-end relation thereto, but the shaft supporting construction involves the following problems.
FIG. 5 shows an example of conventional transmissions in which an input shaft 7 is rotatably supported by casing 4, with a second speed shaft 14 disposed coaxially with the input shaft. The second speed change shaft 14 is fitted at its one end in a cavity 33 formed in the end of the input shaft 7 and is rotatably supported by the shaft 7. The other end of the shaft 14 is supported by the casing 4. With such structure, however, the support of the second speed change shaft 14 by the end of the input shaft 7 becomes very unstable, necessitating a boss portion 34 integral with the casing 4 so that an intermediate portion of the second speed change shaft 14 is supported by a bearing 35 mounted on the boss portion 34. Thus the conventional transmission requires provision of a very solid boss portion 34 which makes the casing 4 complex in construction, difficult to make and very heavy.
Furthermore, a transmission of the type described incorporates a speed change gear system including speed change gears 9 and 10 positioned closest to the prime mover to operatively connect the input shaft 7 to a first speed change shaft 11. It is generally practiced to replace these gears 9 and 10 to obtain a desired speed change, but great difficulties are encountered in replacing the gears 9 and 10 since both the gears 9 and 10 are positioned in the casing 4 according to the conventional structure shown in FIG. 5.